It is highly desirable to inspect the image that will be produced from a printing process prior to the start of on-line printing operations. This is particularly true in color printing processes where both image resolution and color quality must correlate with the original subject matter. An inspection of the proofing image before actual printing of the image provides the operator with an opportunity to adjust the press, plate, ink, film separations, and other conditions to produce a printed product with the desired characteristics.
As is known in the art, color separations can be made from originals, employing red, green and blue filters. The method of producing color separation negatives or positives from the originals is well know in the art. From the original color image, one goes by conventional routes to form positive or negative color separations representing the red, green, and blue record of the original. For the lithographic process, it is of course, required that these records be half-tone renditions. These half-tone separations may be used at this point to form printing plates or pre-press proofs.
Color separations made from the original image may represent a faithful replication of tonal values, and the proofing methods used to make colored representations of these half-tone separations may also reproduce the quality seen in the photomechanically produced originals. However, when these high quality color proofing separations are viewed against the output of a high quality printing press, the results do not always correspond, especially when mid-tone half-tone values are compared. This lack of correspondence is the result of the printing press producing a half-tone dot size on paper that is larger than the half-tone dots present on the printing plate or color separation itself. The net observable result is what is called in the trade as "press gain," produced by the action of a series of moving cylinders spreading out the ink on the final receptor sheet to cover a larger area than is represented by the imaged printing plate or color proofing originals.
These problems have been recognized in the printing trade and certain adjustments, or compensating techniques have been developed to adjust for these variations. However, prior to this invention, there have been no established reproducible processes capable of being accomplished in a non-chemical technical manner with presently available instrumentation for determining the degree of corrections needed.
Color proofing systems are well known and are disclosed in the prior art. The techniques of progressing from an original piece of colored art work through a series of color separations (either negatives or positives), the choice of appropriate color filters to create the specific color rendition on a piece of photographic film, and the use of these separations to generate color proofing separations are well known in the industry.
The methods of combining these individual color separations as well as the means of producing the proper color characteristics are the basis for many patents in this field of photomechanical technology.
U.S. Pat. No. 3,671,236 discloses a pigmented, composite structure on a temporary support capable of being laminated to printing stock, then exposed through a separation negative, and developed to remove the unwanted, nonexposed background sections of the color film. Repeating this process for the other colors comprising the original art work and firmly adhering these colored renditions in exact registration one over the other results in a colored, pictorial representation of the original colored piece.
An important aspect of this invention is the requirement that the clear, colorless pressure sensitive adhesive coated at a dry coating weight of about 200 mg/ft.sup.2 be quite thin in relation to the amounts applied in making conventional pressure sensitive adhesive structures.
U.S. Pat. No. 3,649,268 discloses the use of a photopolymerizable structure which, when laminated and then exposed through a positive separation transparency, may be colored in the unexposed, tacky regions with a preselected toner powder chosen to match the particular separation color and the ink to be used on press. A multi-layer composite structure is prepared by repeating the coloring process for each of the separation positives and then repeating the steps of laminating, exposing and color to produce a single composite sheet.
Both of these disclosures of the prior art requires the use of transparent or at least translucent pigments or toner powders since viewing of all colors in the completed structure requires optical transparency through all the layers.